Microphones are transducers that convert sound into an electrical signal. Microphones are used in a multitude of different applications, such as sound recording, telephones, hearing aids, and various sensor systems. Microphones generally operate most accurately within a particular range of sound levels, depending on a sensitivity and configuration of the microphone. In very loud sound environments, the output signal of the microphone will often become distorted. Particularly, essentially any microphone will have an acoustical overload point (AOP), which is a level of sound at which the microphone can no longer effectively distinguish between the actual sound signal and noise/distortion. For example, the AOP may be defined as the sound pressure level at which distortion in the output signal reaches 10%.
Some types of microphones, such as condenser microphones and capacitive MEMS (microelectromechanical systems) microphones, require a DC bias voltage in order to operate. MEMS microphones additionally require a very high resistance to establish proper DC biasing. This resistance is on the order of few 100's of Giga Ohms.
FIG. 1a shows a microphone circuit 1 for biasing a MEMS microphone 10. The microphone circuit 1 includes charge pump 5 that provides a DC bias voltage for the microphone 10. The circuit 1 includes diodes 25 and 35 which are coupled antiparallel to one another between the charge pump 5 and a node 50. A capacitor 60 is connected between the node 50 and ground. The microphone 10 is connected between the node 50 and a node 40. The microphone 10 modulates the voltage at the node 40 to provide a sensed voltage in response to sound. The circuit 1 further includes diodes 20 and 30 which are coupled antiparallel to one another between the node 40 and ground. Finally, the circuit 1 includes a pre-amplifier 70 having an input connected to the node 40, which provides an output signal at an output node 80 based on the sensed voltage.
One disadvantage of the circuit 1 is that the sensed voltage at the node 40 often has an undesired DC offset. Particularly, due to parasitic resistance Rparasitic of the microphone 10, a small leakage current flows from the node 50 to the node 40, through the microphone 10. The leakage current then flows from the node 40 to ground, through the diodes 20, 30. As a result of the leakage current, the sensed voltage may have a shifted DC offset. For example, the DC offset for the sensed voltage may shift slightly by approximately 300 mV.
Another disadvantage of the circuit 1 is that, at high signal levels, the diodes 20, 30 will clip the sensed voltage, which greatly reduces the AOP of the circuit. Particularly, each of the diodes 20, 30 has a forward voltage VF (e.g., 700 mV) at which it will turn on. At high signal levels, the diodes 20, 30 start to turn on, which distorts the sensed voltage. When the sensed voltage falls below −VF, the diode 20 will turn on and clip the sensed voltage. Similarly, when the sensed voltage rises above +VF, then the diode 30 will turn on and clip the sensed voltage.
FIG. 1b shows an exemplary waveform 90 for the sensed voltage at the node 40 of the circuit 1 in response to microphone 10 being subjected to a high SPL 20 Hz acoustical signal. As can be seen, the waveform 90 is distorted (clipped) when the signal level is too high, due to the diodes 20, 30 being turned on. As is apparent, this clipping effect caused by the turning on of the diodes 20, 30 greatly limits the AOP of the microphone circuit 1. FIG. 2 shows a plot illustrating a frequency spectrum 95 of the waveform 90. As can be seen, the frequency spectrum 95 includes a spike at 20 Hz, which corresponds to the actual sound (i.e. the 20 Hz acoustical signal). However, as can also be seen, the frequency spectrum 95 further includes additional large spikes at 40 Hz, 60 Hz, 80 Hz, 100 Hz, 120 Hz, 140 Hz, and 180 Hz, which correspond to the distortion introduced by the turning on of the diodes 20, 30. As is apparent, this clipping effect caused by the turning on of the diodes 20, 30 greatly limits the AOP of the microphone circuit 1.
One configuration that can reduce the distortion effect includes arranging series stacks of the diodes 20, 30 to provide more headroom for the sensed voltage. This modification increases the AOP of the microphone circuit, but has disadvantages. Particularly, this configuration provides reduced effectiveness at higher temperatures (due to a reduction of forward voltage VF at higher temperatures) and may cause tones in the output signal at normal operation. Another configuration that can increases the AOP of the microphone circuit includes a microphone 10 that is configured with reduced sensitivity. The circuit employs electronic gain to compensate for the reduced sensitivity of the microphone. However, this configuration has the disadvantage of consuming more power. A further configuration that can increase the AOP of the microphone circuit is one in which the gain of the microphone is reduced when high sound levels are detected. However, this configuration has the disadvantage of creating acoustical artifacts, such as clicks and pops, in the output signal. Yet another configuration that can increase the AOP of the microphone circuit is one in which the microphone has multiple membranes with differing sensitivity. The circuit switches between multiple membranes depending on sound levels. However, this configuration also has the disadvantage of creating acoustical artifacts in the output signal.
Accordingly, what is needed is a microphone biasing circuit that achieves a high AOP with high energy efficiency and without introducing acoustical artifacts into the output signal.